Sudoku solver with steps

A solver that teaches

Most sudoku solvers show you the finished grid. This one shows you how to get there. It works like a careful human solver: at every step it looks for the simplest technique that makes progress, applies it, and writes down what it did, such as “Hidden single: 4 can only go in r3c5 in box 2” or “X-Wing on 7 in rows 2 and 6: removes 7 from r4c3, r8c3”. Press Next to watch each step happen on the board, with the cells involved highlighted, and Previous to go back.

Cells are named by row and column: r3c5 means row 3 (counting from the top), column 5 (counting from the left). Boxes are numbered 1 to 9 from the top-left, left to right. Technique names link to our strategy guides where one exists.

How to enter a puzzle

• Type it in: click or tap a cell, then type a digit or use the pad. Arrow keys move between cells. Leave empty cells blank.
• Paste it: paste 81 characters, row by row, using 1-9 for givens and 0 or a full stop for empty cells. Spaces and line breaks are ignored, so a grid copied as nine lines works too.

Before solving, the grid is checked. If the same digit appears twice in a row, column or box, those cells are marked so you can fix the typo. If the puzzle has no solution, or more than one, the solver says so instead of pretending. A valid sudoku needs at least 17 givens: McGuire, Tugemann and Civario proved in 2012 that no 16-clue puzzle has a unique solution, so a grid with fewer than 17 givens is flagged as well.

Which techniques it uses

The solver knows the same fifteen techniques our grader uses for the daily puzzles, in order of difficulty: hidden and naked singles; pointing pairs, box/line reduction, naked and hidden pairs; naked and hidden triples, X-Wing and XY-Wing; Swordfish, XYZ-Wing, simple coloring, unique rectangles (type 1) and Jellyfish. Every puzzle on this site can be solved with these. Some very hard puzzles from elsewhere need more advanced ideas such as chains. In that case the solver explains as many logical steps as it can and then finishes with backtracking, a systematic search that tries a digit and backs up when it reaches a contradiction. That final step is clearly labelled “found by trial and error” so you know it is not a logical deduction.

Using the steps to learn

If you are stuck on a puzzle from a book or newspaper, enter it, press Solve and step forward only until the solver does something you had not spotted. That is the technique to learn next. Reading the explanation alongside the highlighted cells is often enough to recognise the pattern next time. For practice, each strategy guide links to puzzles that cannot be solved without that technique.

Everything runs in your browser. Your puzzle is not uploaded or stored, and solving normally takes a fraction of a second; if a grid takes longer than three seconds, the solver stops and asks you to check for typos.